The present invention relates to a process for manufacturing Al-Mg alloy sheets. More particularly, the present invention is directed to a process for manufacturing Al-Mg alloy sheets suitable for press forming auto body panels, air cleaners, oil tanks and similar products which require superior strength and high formability.
The present invention is also directed to high Mg content Al-Mg alloy sheets which are superior in strength and formability.
One example of prior art uses cold rolled steel sheets for press forming auto body panels and similar products. In recent years, a demand for lighter auto body panels has become popular. Customer demands for lighter automobiles with increased fuel consumption has created a need for lighter auto body panels.
Therefore, it is desirable that the prior art cold rolled steel sheets be replaced with light weight Al-Mg alloy sheets with superior strength and high formability. It is thought that the use of a high Mg content Al-Mg alloy sheet will not only reduce the overall weight of auto body panels, but also contribute to improved fuel consumption.
Prior art aluminum alloy sheets for press forming which exhibit strength and formability, include O stock Al-Mg alloy 5052 which consists essentially of a chromium alloy containing 2.5 wt. % of Al and 0.25 wt. % of Mg. Another example of a prior art aluminum alloy sheet is an O stock Al-Mg alloy 5182 which consists essentially of a manganese alloy containing 4.5 wt. % of Al and 0.35 wt. % of Mg. Further examples include a T4 stock of Al-Cu alloy 2036 consisting essentially of a magnesium alloy containing 2.6 wt. % of Al, 0.25 wt. % of Cu and 0.45 wt. % of Mn.
Of these prior art aluminum alloy sheets, only the Al-Mg alloy sheets exhibit superior formability and strength. Such prior art aluminum sheets are often used due to their capability to adhere to the strict press forming conditions.
Prior art Al-Mg alloy sheets for press forming are usually manufactured by a process which includes forming slabs for rolling, homogenization the slab, followed by hot rolling the homogenized slab, cold rolling and final annealing.
Additionally, an intermediate annealing step may be included prior to the cold rolling step. In situations requiring flat sheets, a straightening step is often carried out by one of a tension leveler, a roller leveler and skin pass rolling after the annealing step.
Conventional Al-Mg alloy sheets for press forming manufactured by such prior art methods are relatively abundant in ductility when compared to other aluminum alloy sheets.
However, the elongation of prior art Al-Mg alloy sheet is approximately 30% at most, whereas the elongation of a cold rolled steel sheet is 40%. Therefore, particularly with respect to the formability, where the elongation is an influencing factor in stretch forming, bending and flanging, the prior art Al-Mg alloy sheet is inferior to the cold rolled steel sheet.
Elongation of Al-Mg alloy sheets can substantially be improved in proportion to the Mg content therein. In order to overcome the above mentioned drawback, prior art methods for producing Al-Mg alloy sheets with improved elongation have attempted to provide a method in which the Mg content is substantially increased.
In one prior art example, the Mg content ranges from 2.5% to about 5.0 wt % which allegedly improves the elongation of the Al-Mg sheets.
In another example of the prior art, a method for producing improved Al-Mg alloy sheets is disclosed, wherein elongation is substantially increased to about 35% when the Mg content is substantially equal to 6 wt. %.
Japanese Laid Open Patent Publication No. 4-102456 attempts to improve elongation by disclosing an Al-Mg alloy sheet with a Mg content of 8%. The presence of this amount of Mg is believed to improve the elongation to about 40%.
One drawback to prior art methods of producing high content Al-Mg alloy sheets on an industrial scale is the appearance of cracks in the alloy material. It has been observed that cracks are often generated during the step of hot rolling. This feature, in turn, makes it difficult to perform subsequent hot rolling of the prior art alloy slabs.
The gist of the drawbacks associated with producing high content Mg Al-Mg alloy sheets is that continuous hot rolling produces cracks, which substantially lowers the yield of the high Mg content Al-Mg alloy sheets and is not cost effective.
In order to substantially improve the output of high content Mg Al-Mg alloy sheets and lower the cost associated with its production, it is necessary to remove the cracked portions as they are generated.
The present invention has been devised to solve the aforementioned problems.